Method for measuring the profile of a road

ABSTRACT

The method of reconstituting the profile of a pavement ( 3 ) consists in:  
     moving three contactless distance-measuring sensors (C av , C mi , C ar ) over a pavement ( 2 ), the sensors being equidistant and in horizontal alignment in the direction of motion, said sensors delivering signals representative of their respective heights above the pavement;  
     measuring the distance travelled by the sensors; and  
     measuring twice the height (H2) measured by the middle sensors (C mi ) from the sum of the heights (H1, H3) of the end sensors (C av , C ar ).  
     The apparatus comprises a horizontal beam fitted with three sensors, a device for measuring the distance travelled, and a computer, the assembly being mounted on a load-carrying chassis or vehicle.

[0001] The present invention relates to the field of measuringdepartures from planeness in the surfaces of road and highway pavements,and of all paths on which vehicles of any type travel, includingrunways.

[0002] Departures from planeness in road or highway pavements, intraffic paths of all types, and in runways, give rise to significantdrawbacks for users and also for the works themselves. For users,numerous studies have shown that the comfort, safety, and costs of usingvehicles are influenced to a very great extent by the vibrations inducedby departures from planeness. So far as the works themselves areconcerned, these defects give rise to additional stresses which shortentheir lifetime.

[0003] As a result, regulations require minimum quality standards to besatisfied when the works are constructed, both for satisfying users andfor ensuring long life for the work. An evaluation of the planenessqualities of a work is also one of the major parameters used duringperiodic inspections thereof for maintenance purposes.

[0004] The advantage of having means for measuring departures fromplaneness is therefore manifest, both for contractors and forauthorities.

[0005] In conventional road terminology, it is the practice to use theterms “profile” and “departures from profile” rather than “departuresfrom planeness”, and apparatus capable of providing an image of the realprofile of the road surface by sampling along one or more substantiallyparallel lines in a given direction, and capable of being included inordinary traffic, is referred to as a “dynamic” profilometer, ascontrasted with “static” profilometers which require the road under testto be closed to traffic.

[0006] It should be observed that all existing profilometers give animage that approximates to the real profile, firstly because they do notobserve the entire surface but only a finite number of lines, andsecondly because they filter the real profile, deforming it both inamplitude and in phase within wavelength bands where their responsediffers from unity, and generally in phase even in frequency bands wheretheir amplitude response is indeed unity.

[0007] So far as roads are concerned, the following are generallydistinguished:

[0008] microtexture for wavelengths shorter than 0.5 millimeters (mm);

[0009] macrotexture for wavelengths lying in the range 0.5 mm to 50 mm;

[0010] megatexture for wavelengths lying in the range 50 mm to 0.5meters (m); and

[0011] smoothness (or conversely roughness) for wavelengths lying in therange 0.5 m to 50 m.

[0012] Present dynamic profilometers can be classified in two broadcategories:

[0013] profilometers using an inertial reference making use of aninertial type artificial horizon as a reference plane, and measuringvariations in height relative to said reference plane in order toestimate profile; by construction such devices are sensitive tomeasurement speed and to the quality of their reference plane; and

[0014] profilometers using a pure geometrical reference, which startingfrom a known position enable profile to be reconstructed by moving aruler with precision; by construction, these devices are sensitive tothe precision with which the ruler is moved and also to measurementerrors, where the influence of such errors generally increasesexponentially with distance.

[0015] The state of the art is illustrated by document WO 98/24977published on Jun. 11, 1998 which shows a profilometer on board avehicle, the profilometer having three contactless distance-measuringsensors mounted at the front of the vehicle chassis and alignedtransversely in a direction perpendicular to the travel direction of thevehicle, together with a system for measuring the positions of thesensors relative to an artificial horizon, said system comprising inparticular an accelerometer for measuring vertical acceleration andinclinometers for measuring the inclinations of the chassis relative tothe artificial horizon, both in terms of roll and in terms of pitch.Each sensor provides a measurement of its height above the pavement. Byusing a computer that is connected to the various devices, thatprofilometer makes it possible to reconstruct the profile along threelines drawn along the pavement, one line to the right of the vehicle,one line to the left of the vehicle, and a central line.

[0016] U.S. Pat. No. 4,571,695 describes a device whose intended purposeis to measure the smoothness of a pavement, i.e. its deformation in theabsence of any load relative to an ideal surface, and it also seeks tomeasure pavement deflection, i.e. deformation under the effect of a loadrelative to its state in the absence of load.

[0017] Given the principle on which it works, the device described inU.S. Pat. No. 4,571,695 requires four sensors referenced 10, 20, 30, and40 in its FIGS. 1 and 2. That document describes measuring smoothnesswith the help of a memory system, requiring extreme accuracy in thepositioning of one measurement relative to another. The term “memorysystem” is used to designate a measurement system in which the value ofmeasurement n depends on the value of measurement k where k<n. Suchsystems present at least two particular features: firstly, any error inmeasurement k induces an error in measurement n and entrains errorpropagation, and secondly it is generally necessary to make assumptionsabout the first measurement or to apply a posteriori corrections on theset of measurements, even if they do not include any error, in order tocompensate for the lack of any antecedents for the first measurement.Thus, in the measurement method described in U.S. Pat. No. 4,571,695,the height of each measurement point is a function of previouslymeasured points and the pitch at which measurement points are sampled isdetermined by the relative position of the various sensors along thebeam which they use as a support.

[0018] The present invention thus seeks to provide a method ofreconstituting the profile of a line drawn on a pavement that makes itpossible to ignore the oscillations of the support for the measuringdevices (body movements if the support is a road vehicle), variations inspeed, speeds of the support, and problems of phase, of the influence ofthe shape of support beam on the sampling pitch, and of the need to usethe preceding points in order to calculate the current point.

[0019] The method of the invention is characterized by:

[0020] moving over the pavement three contactless distance-measuringsensors that are equidistantly in horizontal alignment in the directionof motion;

[0021] simultaneously measuring the height of each of the three sensorsabove the pavement;

[0022] measuring the distance travelled by one of said sensors; and

[0023] substracting twice the height measured by the middle sensor fromthe sum of the heights measured by the end sensors.

[0024] It can be shown by calculation that the result of the subtractionis proportional to the function that represents the profile, and that itis independent of the position of the artificial horizon used inconventional methods of calculation. This is shown below in the presentspecification. In addition, the coefficient of proportionality does notinclude a phase term. As a result, if a direct Fourier transform isapplied to the signal representative of the result of the subtraction,and if a simple multiplying coefficient is applied to the real andimaginary portions of the transform, then the initial profile can beobtained by performing the inverse Fourier transform.

[0025] The three contactless measurement sensors preferably pick up thedistance between themselves and the pavement simultaneously. Thisoperation is repeated each time the sensors have travelled through aselected distance. This distance is fixed for any one series ofmeasurements.

[0026] The travel distance pitch is fixed for a series of measurementscorresponding to a sample or to a portion of the pavement, but thistravel distance pitch can be modified at will. It can be made longerwhen it is desired to measure the smoothness or the megastructure of thepavement, or shorter when it is desired to measure the microtexture orthe macrotexture of certain lengths of the pavement.

[0027] The contactless distance-measuring sensors are preferably of thelaser type using a triangulation principle or a method based ondefocusing, as explained in EP 0 278 269. It is also possible toenvisage using ultrasound sensors operating at high frequency orconventional telemetry devices of precision enabling resolution of about10 microns to be obtained.

[0028] The invention also provides apparatus for implementing themethod.

[0029] The apparatus is characterized by the fact that it comprises:

[0030] a carrying vehicle suitable for being moved along the pavement;

[0031] a longitudinal beam carried by said vehicle in such a manner asto be substantially horizontal;

[0032] three contactless distance-measuring sensors that are mountedequidistantly in horizontal alignment on said beam and that are suitablefor delivering signals representative of their heights above thepavement;

[0033] a device for measuring the distance travelled by the vehicle; and

[0034] a computer receiving signals from the device for measuring thedistance travelled by the vehicle and from the contactlessdistance-measuring sensors.

[0035] Because of the principle on which calculation is based, theproposed apparatus does not introduce any phase distortion in profilemeasurement. As a result it enables the true profile to be reconstitutedeasily by using simple signal processing methods.

[0036] The proposed apparatus does not use an inertial reference. It canthus easily be used in traffic at varying speed, e.g. in an urban area,without that affecting the result of the measurements taken.

[0037] The proposed apparatus is not of the type having a puregeometrical reference. It is thus less sensitive to measurement errorsand less demanding concerning the quality of the distance referenceused.

[0038] Since the proposed apparatus uses contactless sensors anddelivers results that are independent of the movements of its carryingapparatus, it can be used during the operations of building thestructures mentioned in the introduction.

[0039] The proposed apparatus is equally suitable for dynamicallymeasuring smoothness and megatexture, or alternatively staticallymeasuring microtexture and macrotexture.

[0040] It should be observed that the carrying vehicle can be thechassis of a conventional road vehicle.

[0041] Other advantages and characteristics of the invention appear onreading the following description given by way of example and made withreference to the accompanying drawings, in which:

[0042]FIG. 1 shows the general principle on which the calculation methodof the invention is based;

[0043]FIG. 2 is a diagram of the profilometer implementing the method ofthe invention; and

[0044]FIG. 3 shows a vehicle fitted with the FIG. 2 profilometer.

[0045]FIG. 1 shows a horizontal beam 1 which is moved in the direction xdefined by the axis of the beam over a pavement 2 which includesdepartures from planeness, the beam being at a mean height H from thepavement.

[0046] Three contactless distance-measuring sensors are mounted on thebeam 1 and are referenced from the front to the rear of the beam 1 asfollows: C_(av), C_(mi), and C_(ar). Each of the front and rear sensorsC_(av) and C_(ar) is placed at a distance L from the middle sensorC_(mi). The length of the beam 1 is thus at least 2L.

[0047] In conventional manner, each sensor C_(av), C_(mi), and C_(ar)preferably comprises a device for transmitting signals towards thepavement 2, a device for receiving the echo reflected by the pavement 2,a device for measuring the time interval between signal transmission andecho reception, and a device for computing the height of the transmitterabove the pavement 2. An example of a sensor of this type is describedin EP 0 278 269.

[0048] Let the profile of the pavement be a sinewave of equation g(x),where x is the abscissa value for the middle sensor C_(mi).

[0049] Let λ be the wavelength of the sinewave g(x), thus:

g(x)=sin(2πx/λ)

[0050] Let H1, H2, and H3 be the respective heights of the sensorsC_(ar), C_(mi), and C_(av) above the pavement.

[0051] Then:

H1=H−(sin(2πx−L)/λ)

H2=H−(sin(2πx/λ)

H3=H−(sin(2πx+L)λ)

[0052] Writing A=H1+H3−2H2, then:

A=2 sin(2πx/λ)(1−cos(2πx/λ))

A=2(1−cos(2πx/λ))g(x)

[0053] Ignoring a weighting coefficient, the equation for A is theequation of the function g(x).

[0054] It is important to observe that the coefficient does not includeany phase term. As a result, if a direct Fourier transform is applied tothe signal A, and if a simple multiplying coefficient is applied to thereal and imaginary portions of the transform, then the initial profilecan be obtained by performing the inverse Fourier transform.

[0055] If space sampling is performed at a pitch P, and if a directFourier transform is performed on N samples, then point i of thetransform is associated with spatial frequency:

f(i)=i/NP

[0056] however

f(i)=1/λ

[0057] so the multiplying coefficient is given by:

k(i)=1/(2−2 cos(2πL i/NP))

[0058] In the above, it should be observed that the travel speed of thebeam does not appear. The method is therefore independent of speed,which makes it possible to apply the method to a profilometer carried bya vehicle which can be included in any traffic flow.

[0059] In the equation for A, the height H of the beam 1 above thepavement does not appear.

[0060] In the method, the beam 1 can be moved vertically without thatharming the results obtained. It suffices that the beam 1 remains in ahorizontal position.

[0061] During measurements, the three sensors C_(av), C_(mi), and C_(ar)are controlled by a computer so as to pick up simultaneously thedistance between each of them and the pavement.

[0062] To reconstitute the profile of a pavement 2, a point of origin isdetermined form the abscissa x, the distance x travelled by the middlesensor C_(mi) is measured by means of a known device, e.g. a pedometer,and the distance travelled is subdivided into segments of pavement. Ineach segment of pavement, N measurements of the height H1, H2, and H3are performed with sampling at a fixed pitch P, and for eachmeasurement, the value of A is calculated.

[0063] When N measurements have been performed, the profile of thecorresponding segment is reconstituted by means of a computer andsignal-processing programs.

[0064] The pitch P is a constant for a given segment, i.e. for Nsamples. However the pitch P can be modified when changing pavementsegment.

[0065] The weighting coefficient which is the inverse of the multiplyingcoefficient k(i) becomes zero if L is a multiple of λ. It is thereforeimpossible, in theory, to see wavelengths k that are integersubmultiples of L. However, this problem is of no importance, since ifspatial sampling is used, then the weighting coefficients become zerowhen L=k NP/i, k being an integer. It thus suffices in theory to give La value that is irrational in order to avoid the problem. In practice,it suffices to give L a length that is sufficiently short compared withthe wavelengths under investigation to avoid meeting the problem.

[0066] The weighting coefficient decreases with λ, once λ is greaterthan 2L. For λ=100 L, the weighting coefficient is equal to 0.004, i.e.if it is desired to measure millimeter distances, then it is necessaryto have sensors capable of measuring micron distances. In practice, thisconstraint is weaker that it appears insofar as the method is intendedfor measuring road profiles, having spectral characteristics that aresuch that amplitudes corresponding to long wavelengths are much greaterand do not require accuracy of millimeter order. Nevertheless, it isclear that at this level the method departs from the real profile,however the distortion relative thereto is compression of amplitudeswhich is less troublesome, for interpretation purposes, than is phasedistortion.

[0067] The calculations performed above show that the mean height H ofthe beam 1 above the pavement has no influence on the measurementsproviding the beam 1 is horizontal. Otherwise, it is necessary to put aconstraint on height. In practice, it suffices for the height H to besubstantially constant.

[0068] It can be shown that when the sensors are rigidly secured to thebeam 1, then oscillations of the beam give rise to variation in thesampling pitch which has no practical influence on the spectrum obtainedby the direct Fourier transform. When the sensors remain vertical andthe angle of tilt of the beam 1 is statistically zero, and when thewavelength λ is continuous and of constant amplitude, then the energy ofthe spectrum remains the same as with a horizontal beam.

[0069]FIGS. 2 and 3 show apparatus 10 enabling the profile of a pavementto be reconstituted.

[0070] The apparatus essentially comprises a carrying vehicle 11, a beam1 fitted with three equidistant sensors C_(av), C_(mi), and C_(ar), acomputer 12, and a device 13 for measuring the distance travelled by theapparatus 10;

[0071] The nature of the carrying vehicle 11 is of little consequenceexcept that it must be capable of moving together with the beam 1, thecomputer 12, and the device 13 for measuring the distance travelled overstructures of the kind specified in the introduction, roads or highways,and it must be capable of doing so at speeds that are comparable to thespeeds of ordinary users without impeding them or constituting or anyparticular danger for them. It is entirely possible for this purpose touse a vehicle of the minibus or light van type with special bodywork andprovided with the regulation signalling required for dynamic measuringunits.

[0072] The beam 1 is rigid and connected to the carrying vehicle 11 viaa hinge 14 making it possible firstly to remain in a vertical planeparallel to the travel direction of the carrying vehicle 11, andsecondly to remain horizontal using a servo-control device. Thestiffness of the beam 1 can be obtained either by giving it anappropriate shape, or by using materials that present very highintrinsic stiffness, e.g. carbon/kevlar, or special steels, or else bycombining the two above solutions.

[0073] In order to ensure that the beam 1 remains in a vertical plane,it is possible to use the force of gravity and a shaft 15 resting onbearings oriented relative to the longitudinal axis of the carryingvehicle 11, together with damping means 16 and a system for compensatingcentrifugal forces while turning.

[0074] The beam 1 can be kept horizontal by an inertial servo-controldevice or by any other equipment using gravity at the site in questionas a reference.

[0075] The computer 12 is connected to the sensors C_(av), C_(mi),C_(ar), and to the device 13 for measuring the distance travelled. Thesensors operate simultaneously to pick up the height distances betweeneach of them and the pavement at a travel distance pitch which is fixedfor a series of measurements so as to enable the computer 12 toreconstitute the profile of the pavement.

[0076] The sensors can be of the laser type using a triangulationprinciple or using a method based on defocusing. It is also possible toenvisage high frequency ultrasound, or ordinary precision telemetrydevices, that enable resolution of about 10 microns to be obtained.

[0077] The computer 12 performs the following functions: acquiringsignals coming from the device 13 for measuring the travel distance,acquiring and possibly digitizing the signals from the sensors C_(av),C_(mi), and C_(ar) as a function of the travel distance signals providedby the device 13, and reconstituting the profile of the structure. Thesefunctions are performed using a set of appropriate algorithms andprograms.

[0078] The hardware constituting the computer 12 can be based oncommercially available components or on a DSP type processor. Thecomputer power that is strictly necessary is less than that availablefrom a bottom-of-range Pentium II™.

[0079] The device 13 for measuring the distance travelled must deliversignals to the computer 12 that enable it to trigger acquisition at aknown measurement pitch P. It is possible to use a fifth-wheel typedevice or a coder mounted on the gear box of the carrying vehicle andassociated with suitable electronics. The use of a Doppler effect sensoris not recommended if it is desired to be able to perform measurementsat low speeds.

[0080] Assuming a sampling pitch P of 2.5 centimeters (cm) andcalculating a Fourier transform on the basis of 8192 points, then thedistance travelled for this series of measurements is 204.8 meters (m).Assuming that the vehicle carrying the apparatus is travelling at aspeed of 20 meters per second (m/s), then there are 10 seconds (s)available for performing the Fourier transform. On a PC compatiblefitted with a Pentium 90, the time required to perform both transformsis less than 2 s.

[0081] The following tables give results obtained with a simulationprogram.

[0082] The simulation was performed under the following conditions:

[0083] L=0.33 m, sampling P=0.1 m;

[0084] the road profile was simulated using spectral characteristicsanalogous to those of a real road and limited to wavelengths lying inthe range 0.7 m to 44.8 m;

[0085] a single sample of 8192 points was used with weighting by meansof a Hanning window;

[0086] energy was computed by directly summing the squares of themoduluses of the components of the Fourier transform (withoutweighting), and only the five most significant figures are given, soenergies are not comparable for different wavelength ranges, but onlywithin any one range;

[0087] the mean error relative to the profile is equal to the squareroot of the sum of the squares of the point-to-point errors divided bythe number of points;

[0088] computations were performed with precision of about 18significant digits; and

[0089] four situations were treated: the real profile; the horizontalbeam; the purely oscillating beam with vertical sensors; and theoscillating beam with sensors connected to the so-called “real” beam:“Infinite” measurement precision LW energy MW energy SW energyDifferences Real 12244 36406 10368 0.0002 profile Horizontal 12314 3641910368 0.0339 beam Pure oscil- 12271 36483 10417 0.0396 lating beam“Real” beam 12220 36482 10409 1.0036

[0090] Measurement precision 0.002 mm LW energy MW energy SW energyDifferences Real 12244 36399 10365 0.0002 profile Horizontal 12191 3603610268 0.3712 beam Pure oscil- 12155 36115 10316 0.9211 lating beam“Real” beam 12103 36118 10311 1.7437

[0091] Measurement precision 0.02 mm LW energy MW energy SW energyDifferences Real 12216 36345 10335 0.0002 profile Horizontal 11192 330469415 4.123 beam Pure oscil- 11135 33176 9442 3.662 lating beam “Real”beam 11126 33288 9450 3.910

[0092] Measurement precision 0.05 mm LW energy MW energy SW energyDifferences Real 12172 36241 10304 0.0004 profile Horizontal 10235 286488141 11.46 beam Pure oscil- 10227 28392 8206 7.80 lating beam “Real”beam 9807 28834 8135 6.84

[0093] From an initial analysis of these tables, it can be seen that:

[0094] the results obtained with “infinite precision” are entirelycompatible with the theoretical approach thus tending to prove thevalidity of the technique;

[0095] if it is desired to perform pure profile measurement, it isappropriate firstly to have measurement precision of at least 0.002 mm,and secondly to operate under conditions in which the beam ishorizontal. Technologically, such conditions can be achieved, eventhough they are expensive; and

[0096] certain results can appear to be surprising, particularly theerrors for precisions of 0.02 mm and 0.05 mm where moving beams givebetter values than the horizontal beam, and this is doubtless due to thenature of the simulation in which tilt is random and the variationscompensate for resolution.

[0097] If attention is paid to energy measurements only, it can be seenthat the LW (long wave) energy as measured by the beam is very close tothe theoretical energy, which can be interpreted as meaning that thelength of the beam could be shortened further without affecting itsperformance, enabling it to move down to the megatexture range.

[0098] It should also be observed that although the measured energylevels and the real energy levels appear to be rather different, interms of smoothness score, i.e. the logarithms of these energy levels,the differences are of percentage order for measurement precision of0.02 mm, so it would appear that the apparatus is suitable forevaluating smoothness in terms of score using sensors that arecommonplace in metrology.

[0099] It is clear that these results differ from the reality they aresupposed to measure; as mentioned above, the content of the simulatedroad comprises, by construction, only wavelengths lying in the range 0.7m to 44.8 m, which is not true of a real road, and it must be acceptedthat the signal input from the sensors needs to be filtered.Nevertheless, since the beam does not of itself contribute any phasedistortion, it is possible to use filters with known phase variation(e.g. linear phase filters) and to correct the signal for phase as wellas correcting it for amplitude in order to reconstitute the real profilein the above-specified range of wavelengths. Consideration could also begiven to sampling at sufficiently small intervals to ensure thatspectrum folding does not disturb measurements in the wavelength bandsused.

[0100] The method applies to the field of smoothness and megatexture fora vehicle travelling at normal speed. It also applies to themacrotexture and microtexture ranges if the vehicle is travelling at aslow speed.

1/ A method of reconstituting the profile of a pavement, by moving threecontactless distance-measuring sensors (C_(av), C_(mi), C_(ar)) over apavement (2), the sensors being equidistant and in alignment in thedirection of motion, and supplying signals representative of theirrespective heights (H1, H2, H3) above the pavement at a travel distancepitch (P) which is fixed for any one series of measurements, measuringthe distance travelled by one of said sensors, and obtaining informationrepresentative of the longitudinal profile of the pavement (2) bysubtracting twice the height (H2) measured by the middle sensor (C_(mi))from the sum of the heights (H1, H3) measured by the end sensors(C_(av), C_(ar)), the method being characterized by: causing the sensors(C_(av), C_(mi), C_(ar)) to be carried by a rigid beam held permanentlyhorizontal, simultaneously measuring the height of each of the sensorsabove the pavement (2) at each travel distance pitch interval (P)independent of the distance between the sensors, and applyingmathematical processing to the information representative of thelongitudinal profile of the pavement by using direct and inverse Fouriertransforms to deduce therefrom the longitudinal profile of the pavement.2/ A method according to claim 1, characterized by the fact that thedistance pitch (P) is modifiable. 3/ Apparatus for implementing themethod according to claim 1 or claim 2, characterized by the fact thatit comprises: a carrier vehicle (11) suitable for being moved over thepavement (2); a beam (1) mounted on said vehicle (11) in such a manneras to be maintained permanently horizontal, regardless of the slope ofthe pavement (2) on which said vehicle is travelling, three contactlessdistance-measuring sensors (C_(av), C_(mi), C_(ar)) are mountedequidistantly and in alignment on said beam (1), the sensors beingsuitable for delivering signals representative of their respectiveheights above the pavement; a device (13) for measuring the distancetravelled by the carrier vehicle; and a computer (12) receiving signalsfrom the device (13) for measuring travel distance and from thedistance-measuring sensors (C_(av), C_(mi), C_(ar)), said computer (12)triggering simultaneous acquisitions by the contactlessdistance-measuring sensors at a known measurement pitch (P) independentof the distance between the sensors, and performing mathematicalprocessing on the results of said height measurements so as to obtainthe profile of the pavement.